This invention relates to current transforming circuits having error-compensation devices.
Exciting currents are, as is well-known in the art, causes of error or inaccuracy in current transformers which is expressed by a combination of a ratio error and a phase angle error. For this reason, many attempts to reduce exciting currents have been made.
One such attempt, for example, is as follows. An auxiliary current transformer sharing a primary winding with a main current transformer is provided to develop a current expressed by a.multidot.I.sub.2, wherein a is an arbitrary numeral larger than 1, and I.sub.2 is the current in the secondary winding of the main current transformer. Provided between the output terminals of the secondary winding of the main current transformer is a series connection of a load impedance Z.sub.L (i.e. a load on the main current transformer) and a feedback impedance Z.sub.FO expressed by: EQU Z.sub.FO =(Z.sub.L /a-1)
A secondary winding of the auxiliary current transformer is connected to a feedback resistor such that a current expressed by I.sub.2 -a.multidot.I.sub.2 flows therethrough.
Now, terminal voltages e.sub.2 appearing across the secondary winding of the main current transformer will be: ##EQU1##
This means that the component of exciting current for generating the voltage appearing on the output terminals of the secondary winding of the main current transformer reduces to be zero, because such terminal voltage e.sub.2 is zero as shown in the above equation. It is known that the existence of exciting current is necessary to generate secondary induced voltage.
In another attempt to render the current transformer error-free, current-voltage converters having substantially zero load input impedance are employed as loads for the current transformer. Such current-voltage converters are, for example, disclosed in pages 232-233 of "Operational Amplifiers Design and Applications" by J. G. Graeme and G. E. Tobby, published by McGraw-Hill.
In this latter attempt, the voltage across the output terminals of the secondary winding of the current transformer is also zero due to substantial short circuiting of such terminals, because the current-voltage converter has substantial zero input impedance.
In these prior art approaches, however, some error still remains. Current transforming circuits with accuracy acceptable for use in electronic watt-hour metering systems, has not been obtained.